I had that happen when I first started hatching, I put it down mostly to humidity issues--it got too high at the end. I had my husband add a fan to ours, and I got an auto-turner so I didnt' have to open it to turn eggs (it's a Hovnabator 1602N, very cheap styrofoam bator) & I went to using low humidity---and I've had great hatches ever since. I also calibrate the hygrometer/thermometer -- two of them, before I set each batch, to be sure they are working correctly - and I use a water weasel & go off of that reading to adjust temps/humidity. I think all of these things helped contribute to successful hatches for me. Keep trying & good luck to you!